1. Field of the Invention
The present invention relates to measuring scattering parameters for a device under test (DUT), such as a power amplifier, which require that the DUT be operating with a modulated drive signal before accurate measurements can be made.
2. Description of the Related Art
The measurement of a device behavior under complex actual operating conditions has become increasingly desirable. In particular, it can be difficult to accurately measure gain and some reflection coefficients of a power amplifier operating under a realistic modulated signal drive. A small signal measurement alone of a power amplifier is generally incorrect since the DUT will not be biased correctly. A fully modulated measurement, however, may require very dedicated equipment, long measurement times for adequate stability, and special calibration techniques.
It has long been known that it is sometimes advantageous to make S-parameter measurements in the presence of other signals, as evidenced in S. R. Mazunder and P. D van der Puije, “Two-signal Method of Measuring the Large Signal S-parameters of Transistors,” IEEE Trans. On MTT, vol. 26, June 1978, pp. 417-420, incorporated herein by reference. Another method for taking measurements in the presence of other signals is the measurement technique termed the “Hot S22” technique. A typical test system setup for the Hot S22 technique is shown in FIG. 1. In the Hot S22 technique, the DUT 1 (usually a power amplifier) is driven to its normal operating point by a power tone signal provided from a power tone signal generator 2 to the input of the DUT 1, while a second smaller probe tone signal is provided to the output of the DUT 1 from a probe tone signal generator 3. An isolator 4 is sometimes provided between the probe tone signal generator 2 and the DUT 1 to keep DUT output power from affecting the probe tone signal generator 3.
Measurements are made from a coupler 4 connected between the second signal generator 3 and the output of the DUT 1. The coupler 4 provides a signal to a receiver 6 which downconverts the signal from the coupler to an intermediate frequency (IF) for measurement. The probe tone signal from generator 3, as reflected from the output of the DUT 1, is coupled by the coupler 4 to the receiver 6. The reflected signal is compared with the signal from the probe tone signal generator 3 to provide an output reflection coefficient measurement S22. Typically the signals from both the power tone signal generator 2 and the probe tone signal generator 3 are both sinusoids and are offset in frequency by at least several IF bandwidths to avoid effects on measurement due to interference between the power tone signal and the probe tone signal in the receiver, although this may not be necessary with specialized instrumentation to separate the signals based on phase behavior.
The Hot S22 measurements technique described are not typical load pull measurements, since the port impedances remain fixed. Without measurements for different loading, useful information about output reflection behavior and stability of the DUT, however, will still be provided for DUTs operating with 50 ohm loads in a 50 ohm environment. As such, the conventional Hot S22 measurement technique described is often used to characterize amplifier subassemblies rather than amplifiers alone.
The S-parameters of a two port device such as DUT 1 characterize how the device interacts with signals presented to the various ports of the device. The measurement for Hot S22 is, of course the S22 S-parameter. The first number following the S in “S22” indicates the number of the port the signal is leaving, while the second number is the port that the signal is being injected into. S12, therefore, is the signal leaving port 1 relative to the signal being injected into port 2. The four S-parameters associated with an exemplary two-port DUT are represented in FIG. 2, where:                S11 is referred to as the “forward reflection” coefficient, which is the signal leaving port 1 relative to the signal being injected into port 1;        S21 is referred to as the “forward transmission coefficient, which is the signal leaving port 2 relative to the signal being injected into port 1;        S22 is referred to as the “reverse reflection” coefficient, which is the signal leaving port 2 relative to the signal being injected into port 2; and        S12 is referred to as the “reverse transmission” coefficient, which is the signal leaving port 1 relative to the signal being injected into port 2.        
An important point about the Hot S22 measurements is that they can be made with a calibrated vector network analyzer (VNA) 7, as illustrated in FIG. 3. The VNA is calibrated to remove uncertainties and provide traceability. With straight power measurements using a scalar test setup, such as with the signal generator 2, coupler 5 and receiver 6, rather than the VNA 7 used to make measurements, uncertainties will not be removed.
In a modem measurement environment with a variety of wide modulation formats and highly optimized power amplifiers, the need for performance data increases. For instance, other S-parameter measurements than S22 might be desirable, since all results under large signal sinusoidal drive or some other type of drive may not be the same. As an example, forward parameters may be affected as well as reverse parameters. Furthermore, a modulated power drive other than large sinusoidal signals might be desirable since average compression behavior of components in the receiver varies as a function of the statistical distribution of an input signal. More common modulation signals such as code division multiple access (CDMA) and wideband CDMA (WCDMA) may be desirable.